1. Field of the Invention
The present invention relates to a method capable of easy screening of a substance having production potential of secretory-type IgA (so called IgA induction potential) which inhibits the binding of microorganisms and allergens to tunica mucosa.
The present invention also relates to a bacterial strain of genus Bifidobacterium, which is screened by the method and which exhibits strong IgA induction potential.
2. Description of the Prior Art
Immunoglobulin, being antibody or protein having a structural and functional relation with antibody, is classified in terms of functional and structural properties into five different classes i.e. IgG, IgM, IgA, IgD and IgE.
IgA among them comprises two subclasses i.e. IgA1 and IgA2. In IgA1, the L chain (light chain) is covalently bonded with the H chain (heavy chain) thereof, while in IgA2 the L chain is bound in S--S bond with each other instead of binding to the H chain. As for the composition ratio regarding IgA1 and IgA2, IgA1 reaches 90% of total serum IgA, and IgA2 reaches 60% of total secretory-type IgA.
The production sites of IgA exists in submucosal plasma cells such as gut-tunica propria and the like, sialaden and mammary gland. In human gut-tunica propria, the number of IgA production cells is far greater than that of IgG production cells and the ratio between them is about 20:1 in remarkable contrast of the ratio of 1:3 (IgA:IgG) in lympho nodes and spleen. IgA in secreted mucus is formed in dimer having a J-chain component and has a secretory component attached thereto which can be observed only in small amount in serum IgA. This component is attached to a dimeric IgA molecule while it is secreted from plasma cells beneath the tunica mucosa of gut and airway into mucus.
Secretory-type IgA in secreted mucus inhibits the binding of highly pathogenic microorganisms and allergens to tunica mucosa. Thus, secretory-type IgA binds food components functioning as allergens to inhibit the absorption thereof through gastric wall, in addition to prevention of infection of pathogenic microorganisms.
The preventive mechanism of antibody is now illustrated in the following inhibitive mechanism of absorption through gastric wall. Antibody directly binds to the surface of microorganisms in order to prevent infection of exotoxin secreted from pathogenic microorganisms. That is, the antibody may directly bind to the microorganisms to exert a variety of effects.
Alternatively, there has been also observed some substances having the potential to process antigen effectively after nonspecific stimulation of antibody production cells, and such property is often called built-in-adjuvanticity.
It is known, for example, that cholera toxin to be produced by Vibrio cholerae, a causative toxin for diarrhea, may affect tunica mucosa intestini tenuis to modify ionic permeability thereof, so that the toxin may induce intestinum tenuis to discharge a great amount of water and electrolytes to lead to the occurrence of diarrhea. It is also known that the cholera toxin B subunit obtained by detoxifying the principle component of the toxin exhibits immunological response such that it permeates into tunica mucosa to facilitate the production of IgA antibody (so-called IgA induction potential), namely, the cholera toxin B subunit functions as adjuvant.
Ninety-nine percent of enterobacterium flora in breast-feeding babies within several weeks after birth is occupied by bacterial strains of genus Bifidobacterium as an enterobacterium which cannot exert any pathogenicity to humans and animals. The above fact has indicated that the bacterial strains may have certain roles in biophylaxis.
Bifidobacterium longum, one of the genus Bifidobacterium, is known to increase total IgA in feces if administered orally.
However, the induction potential of secretory-type IgA has not been comparatively studied yet and an easy method for the investigation therefor has never been established, either.
It has not been completely elucidated yet how effective the enterobacterium may be. It has not been identified specifically whether there may be any difference in IgA induction potential among a variety of bacterial strains of genus Bifidobacterium known not to have any pathogenicity to humans and animals.